This invention relates to a mass spectrometer, such as a secondary ion mass spectrometer, which comprises a quadrupole electrode.
A mass spectrometer has a device axis. The mass spectrometer comprises, along the device axis, an ionizing section in which gas is preliminarily introduced and which is for ionizing particles of the gas to produce ionized particles, a quadrupole electrode for separating the ionized particles in reference to a mass-to-charge ratio to produce mass-separated particles, and a detector for detecting the mass-separated particles. The quadrupole electrode comprises four electrodes arranged about the device axis at a predetermined azimuthal interval. In the manner which will later be described more in detail, the quadrupole electrode is supplied with a pair of high-frequency voltages superposed on a direct-current potential.
In a conventional mass spectrometer of the type described, four kinds of methods are adopted to analyze the mass-to-charge ratio of ionized particles of high axial energy.
In a first one of the four methods, the high frequency voltages have a common adjustable frequency. The ionized particles are radially accelerated with a maximum accelerating energy which is proportional to a square of the frequency. The frequency is raised to achieve a high radial energy. However, the first method is disadvantageous in that a great deal of high frequency electric power is consumed because the high frequency electric power is consumed in proportion to the frequency to the fifth power.
A second one of the four methods is to vary the direct-current potential of the quadrupole electrode so as to render the direct-current potential substantially near to an energy potential of the ionized particles. The second method is advantageous in that only ions of an energy range are analyzed but are disadvantageous in that the mass spectrometer has a sensitivity which is inevitably reduced because all of the ions can not be analyzed.
A third one of the four methods is to use an energy filter known in the art. The third method is advantageous and disadvantageous like the second method.
A fourth one of the four methods is to lengthen the quadrupole electrode. In this case, the quadrupole electrode should be arranged or assembled with a high accuracy. But, it is difficult to insure a necessary accuracy of assembly because the quadrupole electrode of a longer length such as 60 mm or so, makes it difficult to assemble the quadrupole electrode and to support the same. As a result, such a mass spectrometer is not feasible because a resolution of mass spectrum is inescapably degraded with a length of the quadrupole electrode.